The present invention relates to a hydraulic control system for automatic transmissions used in vehicles.
Generally, a conventional automatic transmission for a vehicle includes a torque convertor, a multi-stage gear shift mechanism, and a plurality of friction elements, actuated by hydraulic pressure and which select a gear range of the gear shift mechanism according to a driving state of a vehicle.
A hydraulic control system. for automatic transmissions used in vehicles operates by the selection of friction elements using hydraulic pressure, created in a hydraulic pump, which passes through a control valve. As a result, shifting can be realized automatically, and an appropriate driving state of the vehicle is achieved.
The above hydraulic control system includes a pressure regulator, which regulates hydraulic pressure created in a hydraulic pump; manual and automatic shift controllers, which control shifting; a hydraulic controller, which controls shift quality and shift response for a smooth shift; a damper clutch controller, for operation of a torque convertor damper clutch; and hydraulic pressure distributer, which supplies an appropriate amount of hydraulic pressure to each of the friction elements.
In the hydraulic control system structured as above, through the ON/OFF and duty control of solenoid valves by a transmission control unit, the hydraulic pressure distributer changes the hydraulic distribution and thus friction elements to be operated so as to control the shifting between shift stages.
However, when controlling shifting from the present shift stage to a different shift stage in the above hydraulic control system, the timing required to release hydraulic pressure operating on the present friction element and to supply hydraulic pressure to a new friction element greatly influences shift quality. Also, in the prior art hydraulic control system, engine rpms can quickly increase, the shift mechanism can become interlocked, and the engine can briefly enter a neutral state.
To improve shift quality by controlling the timing of supplying hydraulic pressure, the prior art changes a structure of the shift valves. But this change results in the drawback of complicating the shift valve structure, and if an ECU malfunctions, fail-safe functions can only operate for one shift stage.